Well bores of oil and gas wells extend from the surface to permeable subterranean formations (‘reservoirs’) containing hydrocarbons. These well bores are drilled in the ground to a desired depth and may include horizontal sections as well as vertical sections. In any arrangement, piping (e.g., steel), known as casing, is inserted into the well bore. The casing may have differing diameters at different intervals within the well bore and these various intervals of casing may be cemented in-place. Other portions (e.g., within producing formations) may not be cemented in place and/or include perforations to allow hydrocarbons to enter into the casing. Alternatively, the casing may not extend into the production formation (e.g., open-hole completion).
Disposed within a well casing is a string of production piping/tubing, which has a diameter that is less than the diameter of the well casing. The production tubing may be secured within the well casing via one or more packers, which may provide a seal between the outside of the production piping and the inside of the well casing. The production tubing provides a continuous bore from the production zone to the wellhead through which oil and gas can be produced.
The flow of fluids, from the reservoir(s) to the surface, may be facilitated by the accumulated energy within the reservoir itself, that is, without reliance on an external energy source. In such an arrangement, the well is said to be flowing naturally. When an external source of energy is required to flow fluids to the surface the well is said to produce by a means of artificial lifting. Generally this is achieved by the use of a mechanical device inside the well (e.g., pump) or by decreasing the weight of the hydrostatic column in the production tubing by injecting gas into the liquid some distance down the well.
The injection of gas to decrease the weight of a hydrostatic column is commonly referred to as gas lift, which is artificial lift technique where bubbles of compressed air/gas are injected to reduce the hydrostatic pressure within the production tubing to below a pressure at the inlet of the production tubing. In one gas lift arrangement, high pressure gas is injected into the annular space between the well casing and the production tubing. At one or more predetermined locations along the length of the production tubing, gas lift valves permit the gas in the annular space to enter into the production tubing.
The gas lift valves are supported by gas lift mandrels, which are devices installed in the production tubing onto which or into which the gas-lift valve is fitted. In a conventional gas-lift mandrel, the gas-lift mandrel is a short section of tubing disposed in the production tubing string that supports a gas lift valve disposed on its exterior surface. The gas lift valve controls the flow of pressurized gas from the well casing through a valve port into an interior of the mandrel. Tubing and casing pressures cause the gas-lift valve to open and close, thus allowing gas to be injected into the production tubing causing fluid in the tubing to rise to the surface. Further, different mandrels may have valves with different pressure settings.
Conventional gas-lift mandrels are installed as the production tubing is placed in the well casing. During the placement of the production tubing, the production tubing is commonly filled with fluid such that the production tubing is not buoyant prior to placement of the packer(s). Thus it is necessary to isolate the gas lift valve(s) during production tubing placement to prevent injection of gases in the well casing into the production tubing. Previously, such isolation has entailed the insertion of a frangible sealing disk (e.g., ceramic) at the upper and/or lower joint between the mandrel and the production tubing. Once the production tubing is set within the casing, such frangible seals are removed by, for example, application of pressure and/or lowing of a breaking implement through the interior bore of the production tubing.
One drawback of the use of such frangible seals is that a portion (e.g., peripheral rim portion) can remain within the interior bore of the production tubing. Such remaining portions of the frangible seal may hinder or prevent the insertion of down-hole implements through the production tubing. For instance, such remaining seal portions may prevent passage of a plunger preventing use of plunger assisted gas lift for the well without removal of the entire string of production tubing.